It "could be very soon or not so soon at all", said the president, who has cancelled a planned trip to allow him to stay in the US with his defence secretary, and has been canvassing support for strikes from the leaders of France and the UK.

Also on Thursday, US Secretary of Defense Jim Mattis told a congressional panel: "I believe there was a chemical attack and we are looking for the actual evidence."

Later in the day, the British Cabinet is due to discuss its response and the UN Security Council will hold an emergency meeting on the crisis.

Did Macron offer evidence?

The French leader had previously said any strikes would target the Syrian government's "chemical capabilities".

He did not give the source of his information but said: "We have proof that last week chemical weapons, at least chlorine, were used by the regime of Bashar al-Assad."

Asked in a TV interview whether France would join strikes on Syria, he said: "We will need to take decisions in due course, when we judge it most useful and effective.

"Regimes that think they can do everything they want, including the worst things that violate international law, cannot be allowed to act."

Macron's 'red lines'

Lucy Williamson, BBC News, Paris

A year ago, Russian President Vladimir Putin stood grim-faced in the opulence of France's Versailles Palace, as a newly elected Emmanuel Macron announced that the use of chemical weapons in Syria was a "red line" for France, and "would result in reprisal and an immediate response".

Now Mr Macron - who likes to be seen as a president who does what he says he will do - is facing the test of what "reprisal" actually means.

Former US President Barak Obama was derided in France for not following through on his own "red line" after Syrian attacks five years ago.

"When you fix red lines," Mr Macron told a French newspaper last year, "if you can't enforce them, you decide to be weak."

Strength is an important part of President Macron's bid to project French influence on the world stage - and a key element, too, of his new political friendship with his US counterpart Donald Trump.